The proposed research examines how English readers and listeners use their knowledge of the semantic structure of their language to comprehend sentences and discourse. It builds on earlier research examining how they use syntactic structure, extending it to topics that have been investigated in the field of semantics. In particular, the research investigates how people are able to comprehend sentences with variables, which are an essential source of the expressive power of human languages. It studies three distinct aspects of the processing of variables: a) true vs. 'fake'variable binding ("Every man loves his wife" vs. "The men bought presents for their wives;" b) the processing of `world variables,'essentially implicatures that some state of affairs is not true in the actual world;and c) the determination of the domain over which a variable is quantified (e.g. does "The offices are mostly clean" refer to most offices or to degrees of cleanliness?). Each of these topics is the subject of intensive analysis within the field of formal semantics and is ready for experimental psycholinguistic study. The experiments that are proposed study these topics with a variety of techniques, ranging from simple tasks of judging the meaning or the naturalness of sentences and discourses to measures of eye movements during normal reading. PUBLIC HEALTH RELEVANCE: The immediate goal of the research is to explore how normal adult readers and listeners compose the meaning of a sentence out of the meaning of its parts. This is a skill that is universal among intact humans, but that can be degraded or lost in cases of language disorder, most notably aphasia. A better understanding of aphasia must be based on a better understanding of the specific linguistic functions that are lost in aphasia, and it is hoped that such a better understanding will lead to better treatment - an important goal in an aging population with an increasing expected incidence of strokes leading to language impairments.